High pressure ratio multi-stage centrifugal compressor

ABSTRACT

A heat pump system includes a refrigerant circuit. First and second heat exchangers are arranged in the refrigerant circuit. A flow reversing device selectively changes a direction of flow in the refrigerant circuit between the first and second heat exchangers. A centrifugal compressor is arranged in the fluid circuit and has first and second impellers arranged in series relative to one another to provide a desired compressor pressure ratio. In one example, the centrifugal compressor mounts the first and second impellers on opposing ends of a shaft. The first and second impellers respectively include first and second stage inlets and outlets. One example desired compressor pressure ratio of at least 10:1 corresponds to a second stage outlet pressure to a first stage inlet pressure. A first diffuser is arranged at the first stage outlet, and the first diffuser is a variable geometry diffuser.

BACKGROUND

This disclosure relates to a multi-stage centrifugal compressor for usein a high pressure ratio multi-stage centrifugal compressor having atleast one variable geometry diffuser.

Existing single-stage and two-stage centrifugal refrigerationcompressors, with vaneless or vaned diffusers, typically have at leastone set of variable inlet guide vanes at a compressor inlet to regulatecompressor capacity during various operating conditions.

Variable-speed centrifugal compressors use speed variation as theirprimary capacity control mechanisms, but such compressors still needvariable inlet guide vanes in order to operate surge-free at lowcapacity conditions.

Multi-stage centrifugal compressors have used stages with fixed,typically vaneless, diffusers with inlet guide vanes and variable speedas the capacity control mechanism. Single-stage refrigerant compressorshave been introduced that employ a variable frequency drive (VFD) forcapacity control in addition to a set of rotatable inlet guide vanesupstream of the impeller. In the case of a variable speed capability, avariable-geometry diffuser has been used downstream from the impeller toimprove the compressor surge characteristics at part-load operatingconditions.

Heat pump systems require compressors having high pressure ratios.Typically, screw-type or scroll-type compressors are used provide theneeded high pressures of heat pump systems.

SUMMARY

A heat pump system includes a refrigerant circuit. First and second heatexchangers are arranged in the refrigerant circuit. A flow reversingdevice selectively changes a direction of flow in the refrigerantcircuit between the first and second heat exchangers. A centrifugalcompressor is arranged in the fluid circuit and has first and secondimpellers arranged in series relative to one another to provide adesired compressor pressure ratio.

In one example, the centrifugal compressor mounts the first and secondimpellers on opposing ends of a shaft. The first and second impellersrespectively include first and second stage inlets and outlets. Oneexample desired compressor pressure ratio of at least 10:1 correspondsto a second stage outlet pressure to a first stage inlet pressure. Afirst diffuser is arranged at the first stage outlet, and the firstdiffuser is a variable geometry diffuser.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure can be further understood by reference to the followingdetailed description when considered in connection with the accompanyingdrawings wherein:

FIG. 1 is a highly schematic view of a heat pump system having anexample centrifugal compressor with multiple stages.

FIG. 2 is a cross-sectional view of an example two-stage centrifugalcompressor of this disclosure.

FIG. 3 is a schematic view of an economizer for the disclosedcentrifugal compressor.

FIG. 4 is a schematic view of one example variable geometry diffuser forthe disclosed centrifugal compressor.

DETAILED DESCRIPTION

Referring to FIG. 1, a heat pump system 10 includes a centrifugalcompressor 12 for circulating a refrigerant in a refrigerant circuit 32.The centrifugal compressor 12 is arranged in the fluid circuit 32 andincludes compressor inlet and outlet passages 16, 18 that are in fluidcommunication with a flow reversing device 14. The example heat pumpsystem is exemplary of a high pressure ratio system. It should beunderstood the disclosed centrifugal compressor may be used in otherhigh pressure ratio applications, such as certain air-cooled chiller orrefrigeration systems.

First and second heat exchangers 20, 22 are fluidly arranged in therefrigerant circuit 32 and respectively arranged at first and secondlocations 21, 23, which may be indoors and outdoors in one example. Inthe example, blowers 34, 36 are respectively associated with each heatexchanger for transferring heat between each heat exchanger and itssurrounding environment. It should be understood, however, that althoughrefrigerant-to-air heat exchangers are shown, a heat exchanger may beused that transfers heat between the refrigerant and another fluid, suchas water.

The flow reversing device 14 selectively changes the direction of flowin the refrigerant circuit 32 between the first and second heatexchangers 20, 22. In the example arrangement, an “H” depicts a heatingdirection of refrigerant flow, and a “C” shows a cooling direction ofrefrigerant flow.

A first bypass valve 24 and a first thermal expansion valve 26 areassociated with the first heat exchanger 20. A second bypass valve 28and a second thermal expansion valve 30 are associated with the secondheat exchanger 22. The bypass valves 24, 28 act as check valves topermit flow in only one direction.

In operation, with the flow reversing device 14 actuated to a coolingconfiguration, the refrigerant flows to the first heat exchanger 20where heat is rejected to the first location 21. Refrigerant then flowthrough the bypass valve 24 and is expanded through thermal expansiondevice 30. The refrigerant is vaporized and then enters the second heatexchanger 22 where the second location 23 rejects heat to therefrigerant before being returned to the centrifugal compressor 12. Withthe flow reversing device 14 actuated to a heating configuration, therefrigerant flows to the second heat exchanger 22 where heat is rejectedto the second location 23. Refrigerant then flow through the bypassvalve 28 and is expanded through thermal expansion device 26. Therefrigerant is vaporized and then enters the first heat exchanger 20where the first location 21 rejects heat to the refrigerant before beingreturned to the centrifugal compressor 12. The heat pump system 10 isintended to be exemplary only.

Referring to FIG. 2, the centrifugal compressor 12 includes a housing 39within which an electric motor 38 is arranged. The housing 38 isschematically depicted and may comprise one or more pieces. The electricmotor 38 rotationally drives first and second impellers 42, 44 via arotor shaft 40 about an axis to compress the refrigerant in a two-stagecompressor configuration. The rotor shaft 40 may comprise one or morepieces. Although two compressor stages are shown, the disclosure mayalso be used in a compressor having more stages. In the example shown,the first and second impellers 42, 44 are located on opposing ends ofthe rotor shaft 40. The impellers are centrifugal such that the impellerinlet is arranged axially and the impeller outlet is arranged radially.

An oil-free bearing arrangement is provided for support of the rotorshaft 40 so that oil-free refrigerant can be used in the centrifugalcompressor 12. In the example, the rotor shaft 40 is rotationallysupported relative to the housing 39 by magnetic bearings 46, which areillustrated in a schematic fashion. The magnetic bearings 46 may includeradial and/or axial magnetic bearing elements, for example. A bearingcontroller (not shown) communicates with the magnetic bearing 46providing a magnetic bearing command to energize the magnetic bearings46. The magnetic bearings create a magnetic field levitating the rotorshaft 40 and controls its characteristics during operation of thecentrifugal compressor 12. It should be understood that the discloseddiffuser arrangements may also be used with air bearings or other typesof bearings.

One example electric motor 38 includes a rotor supporting multiplemagnets about its circumference. A stator is arranged about the rotor toimpart rotational drive to the rotor shaft 40 when energized. In oneexample, a motor controller (not shown) communicates with the stator andprovides a variable speed command to rotationally drive the impellers43, 44 at a variable speed depending upon compressor operatingconditions. The motor controller communicates with multiple sensors (notshown) to monitor and maintain the compressor operating conditions.

The first and second impellers 42, 44 are arranged in series relative toone another and providing a desired compressor pressure ratio, which inone example is at least 10:1. The first impeller 42 includes a firststage inlet 48 and a first stage outlet 52, and the second impeller 44includes a second stage inlet 54 and a second stage outlet 58. First andsecond stage volutes 50, 56 are arranged respective at the first andsecond stage outlets 52, 58. The desired compressor pressure ratiocorresponds to a second stage outlet pressure to a first stage inletpressure.

First and second diffusers 60, 62 are respectively arranged at the firstand second stage outlets 52, 58 near the first and second stage volutes50, 56. In one example, the first and second diffusers 60, 62 arevariable geometry diffusers, which may be any suitable type. First andsecond actuators 64, 66 are configured to respectively move the firstand second variable geometry diffusers between first and secondpositions. Referring to FIG. 4, the second compressor stage is shown asan example. A passage 70 is arranged downstream from the second impeller44. A variable geometry device, such as a movable wall 72, for exampleis arranged in the passage in the second stage outlet 58. The secondactuator 62 moves the wall 72 between a first position F and a secondposition S to selectively regulate refrigerant flow through the passage70.

Any number of variable geometry diffuser arrangements may be used. Forexample, a movable-wall variable-geometry diffuser includes a pluralityof fixed wedge shaped vanes located downstream of the movable diffuserwall element. The movable wall element may be serrated with the trailingedge of the serration coinciding with the throat area (which is thesmallest cross-sectional flow passage) of the vaned diffuser.

In another variable geometry diffuser example, a rotatable vane diffuserin the plane normal to the compressor axis. By rotating the vanes, thethroat area of the diffuser changes, and, therefore, the capacity of thecompressor, is adjusted.

In another variable geometry diffuser example, a split vaned diffuserhas an outer diffuser ring and an inner diffuser ring. Throat areaadjustment is obtained by rotating one diffuser ring with respect to theother one. Throat area of the fully opened diffuser is reduced as aresult of the rotation of the inner diffuser ring relative to the outerdiffuser ring.

An economizer 68 may be mounted to the housing 39, as shown in FIG. 3.In one example, the economizer 68 is a heat exchanger-type economizer.The economizer 68 is arranged fluidly between the first stage outlet 52and the second stage inlet 54.

Although two compressor stages are shown in FIG. 2, the disclosure mayalso be used in a compressor having more stages. For the showndirect-drive gearless multistage compressors the impellers are arrangedon opposing ends of the rotor shaft (the so-called back-to-backconfiguration) reducing the axial thrust load of the shaft. In oneexample three-stage configuration, only the first compressor stageincludes a variable geometry diffuser, and the second and third stagesmay each include a volute without a diffuser and still achieve thedesired compressor pressure ratio.

Although example embodiments have been disclosed, a worker of ordinaryskill in this art would recognize that certain modifications would comewithin the scope of the claims. For that reason, the following claimsshould be studied to determine their true scope and content.

What is claimed is:
 1. A heat pump system comprising: a refrigerantcircuit; first and second heat exchangers arranged in the refrigerantcircuit; a flow reversing device selectively changing a direction offlow in the refrigerant circuit between the first and second heatexchangers; and a centrifugal compressor arranged in the fluid circuitand having first and second impellers arranged in series relative to oneanother and providing a desired compressor pressure ratio.
 2. The heatpump system according to claim 1, wherein the first and second impellersrespectively include first and second stage inlets and outlets, and thedesired compressor pressure ratio corresponds to a second stage outletpressure to a first stage inlet pressure.
 3. The heat pump systemaccording to claim 2, wherein the desired compressor pressure ratio isabout 10:1.
 4. The heat pump system according to claim 2, wherein thecompressor includes first and second diffusers respectively arranged atthe first and second stage outlets.
 5. The heat pump system according toclaim 4, wherein the first and second diffusers are variable geometrydiffusers, and the compressor includes first and second actuatorsconfigured to respectively move the first and second variable geometrydiffusers between first and second positions.
 6. The heat pump systemaccording to claim 2, comprising an economizer arranged fluidly betweenthe first stage outlet and the second stage inlet.
 7. The heat pumpsystem according to claim 1, wherein the compressor includes a variablespeed motor configured to rotationally drive the first and secondimpellers.
 8. The heat pump system according to claim 1, whereincompressor includes a shaft supporting the first and second impellers,and magnetic bearings support the shaft.
 9. A refrigerant centrifugalcompressor comprising: first and second impellers mounted on opposingends of a shaft, the first and second impellers respectively includefirst and second stage inlets and outlets, and a desired compressorpressure ratio of at least 10:1 corresponding to a second stage outletpressure to a first stage inlet pressure; and a first diffuser arrangedat the first stage outlet, wherein the first diffuser is a variablegeometry diffuser.
 10. The refrigerant centrifugal compressor accordingto claim 9, comprising a second diffuser arranged at the second stageoutlet.
 11. The refrigerant centrifugal compressor according to claim10, wherein the second diffuser is a variable geometry diffuser.
 12. Therefrigerant centrifugal compressor according to claim 11, comprisingfirst and second actuators configured to respectively move the first andsecond variable geometry diffusers between first and second positions.13. The refrigerant centrifugal compressor according to claim 9,comprising an economizer mounted on the compressor and arranged fluidlybetween the first stage outlet and the second stage inlet.
 14. Therefrigerant centrifugal compressor according to claim 9, comprising avariable speed motor configured to rotationally drive the first andsecond impellers.
 15. The refrigerant centrifugal compressor accordingto claim 9, comprising magnetic bearings supporting the shaft.